This study is designed to investigate certain adaptive changes in muscle tissue associated with alterations in muscle length. Alterations in muscle length occur during growth and can also be induced by dental modifications of the intermaxillary space and by orthognathic surgery. Our primary objective is to understand the structure and function of the muscles of the craniofacial region and to determine the adaptations that occur in the craniofacial musculature with changes in length. The effects of muscle detachment also will be considered. Specific structural and functional adaptations will be produced and investigated by histological, histochemical, biochemical, anatomical and physiological techniques, some of which have not been commonly used in previous studies of craniofacial growth. Data and interpretations gained from this study will be combined with data gained from our previous experimental investigations, leading to a better understanding of musculoskeletal adaptation in the craniofacial region.